Teach-in '99 : Evolution of Bipedal Gait
by Chris Kirtley, The Hong Kong Polytechnic University

One perennial question about bipedal gait is "Why did it evolve?". As this wonderful Rubes cartoon nicely illustrates, there must have been a stage when some humans walked upright and some still used their knuckles!

Rubes cartoon

Or was the transition more sudden? The fossil footprints (see Teach-in #14 on Toe Clearance ) found at Langebaan Lagoon in Africa would seem to suggest that we have been walking the way we do now for a long time. So what could have been the driving force behind this change in locomotion style? It must have been a strong stimulus, in view of not only the speed of transition, but also the additional control and balance difficulties imposed by bipedalism. There must have been a big "pay-off' in evolutionary terms.

There have been several suggestions as to what this selective pressure was:

At any rate, anthropologists seem to be agreed that the transition took place at around the time of great climatic changes in the eastern Rift valley area of Africa, about 7 million years ago (mya). Dense rain-forest was being replaced by savannah, so our ancestors were faced by the problem of how to move from tree to tree across open grassland, pursued, no doubt, by lions and tigers. Darwin thought that bipedalism, technology, and increased brain size came together as a 'package' However, stone tools do not appear in the archaeological record until about 2.5 mya, well after the proposed origin of hominids at 7 mya. This seems to refute the notion that we walk upright to use our hands.

There must therefore have been an evolutionary pressure to develop a fast, efficient and stable locomotion pattern with which to do this... but why bipedal gait? The question is not trivial, since it presumably guided the development of the locomotor system and very probably influences the compensation to gait disorders seen in patients. For example, is the underlying goal of gait...

Inman & Saunders thought that energy efficiency was the guiding principle, when they formulated their famous Determinants of Gait. They thought that minimising the vertical and side-to-side oscillations of the total body centre of mass was the guiding principle of gait. However, recently a lot of these "determinants" have been challenged. So what are we left with - what is the heuristic goal of the nervous system when it is controlling walking?


Saunders, Inman and Eberhart, J. Bone & Jt. Surg., 35-A(3): 543-558, 1953.

S.A. Gard, D.S. Childress (1997) Gait & Posture, 5 (3): 233-238, Investigation of vertical motion of the human body during normal walking.

S.A. Gard, D.S. Childress (1997) Gait & Posture, 5 (2): 161, The effect of pelvic list on the vertical displacement of the trunk during normal walking


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